Kate's Latest Accessory: 'Baby on Board!' Button

The Duchess of Cambridge received an appropriate memento of her trip to a London tube station Wednesday – a "Baby on board!" button.

"I'll make sure I wear this at home," Kate, laughing, said as she was handed the badge, emblazoned with the London Underground logo.

Wearing a Malene Birger coat and black beret, Kate was along for the ride with Queen Elizabeth and her husband Prince Philip. The visit helped mark the 150th anniversary celebrations of the subway network.

Touring the Baker Street station, Kate, 31, revealed she "missed" traveling on the public transport system.

"She said she used the tube on a regular basis before she was married," Howard Collins, chief operating officer of London Underground, tells PEOPLE.

Queen Elizabeth II

Tim Rooke / REX USA

The badges are worn by pregnant women for fun – and practical reasons. "She asked if it worked effectively, and I said it helps save some people the embarrassment of not noticing," said Collins.

The Queen, who wore a summer tweed dress by Karl Ludwig and hat by her in-house designer Angela Kelly, looked a bit frail following her bout with a stomach bug that put her into the hospital. Wednesday's visit was her first public outing since then. It was also the third time Kate had joined the monarch for an engagement.

The royal party met staff and toured the trains before the 86-year-old monarch named a brand new train the Queen Elizabeth II.

Across the station, on other platforms, surprised commuters craned their necks for a view, and many didn't get on passing tube trains so they could stay and watch the royals.